1. The present invention relates to a fixing device capable of favorably removing an electric charge which disturbs a toner image at a time of fixing to cause an image deterioration from a surface of a heating rotary member.
2. Description of the Related Art. A fixing device used in an image forming apparatus such as a copying machine and a printer allows a pressing member to come in contact with a heating rotary member such as a fixing drum, a fixing belt or the like to form a fixing nip portion. Then, the fixing device allows a sheet bearing an unfixed toner image to pass through the fixing nip portion in a manner such that the toner image comes in contact with a surface of the heating rotary member, and then applies heat and pressure to the same to fix the toner image onto the sheet surface.
Since the unfixed toners are electrically charged at this time, they may be gradually accumulated on the fixing rotating member. In such case, the accumulated toners are likely to disturb a toner image formed on a next sheet to be fixed to cause an image deterioration.
Various technologies for preventing such deterioration in an image quality have been proposed. For example, documents 1 through 6 disclose the following contents.
Japanese Patent Unexamined Publication No. HEI 3-116176 (document 1) discloses a state where an earthed charge-removing brush is in contact with a heating roller. Japanese Patent Unexamined Publication No. 2003-280420 (document 2) discloses a charge-removing unit capable of moving toward and apart from a heating member, and Japanese Patent Unexamined Publication No. HEI 3-148682 (document 3) discloses a state where a charge-removing roller made of metal comes in contact with a fixing roller having an elastic rubber layer so that it is rotated. Further, Japanese Patent Unexamined Publication 2002-162857 (document 4) discloses a state where an earthed charge-removing brush comes in contact with an exposed part of a metal core of a fixing roller. Japanese Patent Unexamined Publication No. HEI 3-91786 (document 5) discloses an arrangement of a discharge plate so shaped as to have a plurality of sharp-pointed projections to be positioned close to a surface of a heating roller in a non-contact state. Further, Japanese Patent Unexamined Publication 2005-31402 (document 6) discloses a state where a charge-removing sheet is so arranged as to face a surface of a heating roller on upstream of a fixing nip portion along a rotational direction of a heating roller.
However, according to the above-described documents 1 to 4, a member adapted to remove an electric charge (the charge-removing brush, the charge-removing unit, the charge-removing roller, and the earthed charge-removing brush) is so arranged as to come in contact with a member whose electric charge is to be removed (the heating roller, the heating member, the fixing roller, and the metal core of the fixing roller). Accordingly, there is a problem that a charge-removing efficiency is lowered drastically in a case where the contact part is contaminated by toners and paper powders.
Further in a case where an image forming apparatus is a color image forming machine, and a charge-removing member comes in contact with a sheet-passing area of a surface bearing a toner image (print surface side), a contact line (a line caused by a tight contact) is likely to appear on an image.
On the other hand, the discharge plate as a charge-removing member in the documents 5 and the charge-removing sheet in the document 6 are so arranged in non-contact state with respect to the member whose electric charge is to be removed. Accordingly, the lowering in a charge-removing efficiency due to toners and paper powders and occurrence of the contact line in a color image like the ones described above can be prevented.
However, there is a problem that the charge-removing members disclosed in the documents 5 and 6 can effectively remove an electric charge gradually accumulated in a wide area of a surface of the heating rotary member, but sufficient removal of electric charge is not possible if the electric charge is locally and remarkably accumulated. In other words, a great difference in electric potential occurs between a toner existing part and a toner non-existing part on a sheet at a time of fixing operation. Further, when a rear end of the sheet is separated from the heating rotary member, a separating discharge occurs. The charge-removing members of the documents 5 and 6 require a long time for removal of locally and remarkably accumulated electric charge. Therefore, an image deterioration is likely to occur, for example, when a second or later sheet is fixed at a time of consecutive sheet transfer.